Gastric Secretion

Dr.Mohammed Sharique Ahmed Quadri Assistant Professor Physiology

Almaarefa College

بسم الله الرحمن الرحيم

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Objectives • Describe the three types of gastric exocrine secretory

cells and there secretions.• Summarize the mechanism of HCl secretion in stomach.• Explain the role of HCl and pepsinogen in digestion.• Emphasize the protective role of mucus and its applied

importance.• Describe the multiple regulatory pathways that control

stomach secretion.• Appraise the role of stomach in digestion and

absorption.

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GASTRIC SECRETION

• Gastric Secretion is about 2 liters – per day.• There are Gastric Glands present in the

stomach.• They open at the surface of the stomach by

opening called ‘Gastric Pits’.• Gastric Glands have exocrine and endocrine

cells.

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Gastric Secretions

• Two distinct areas of gastric mucosa that secrete gastric juice–Oxyntic mucosa• Lines body and fundus

–Pyloric gland area (PGA)• Lines the antrum

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GASTRIC SECRETION

• Exocrine cells in Gastric Gland are of three types: 1. Mucus Cells – they secrete thin watery mucus.

2. Chief Cells – they secrete enzyme precursor pepsinogen. Chief cells are present in oxyntic mucosa ( fundus and body )

3. Parietal [Oxyntic cells] – they secrete HCL and intrinsic factor. They are present in fundus and body.

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GASTRIC SECRETION

• All exocrine secretion are released in Gastric lumen and make up the gastric digestive juice.

• Between the Gastric pits, gastric mucosa is covered by surface epithelial cells, which secrete a thick, viscous, alkaline mucus several millimeter thick over the surface of mucosa.

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GASTRIC SECRETIONEndocrine – Paracrine cells in stomach.

• ECL Cells( Entero Chromaffin Cells ) – they secrete Histamine and stimulate parietal cells to produce HCL. They are present in fundus and body [Oxyntic mucosa].

• G–Cells – present in the antrum [Pyloric Gland Area PGA]. They

secrete Gastrin hormone which stimulate parietal, chief and ECL [Entero Chromaffin Like cells].

• D–Cells – present in PGA. They secrete somatostatin and they inhibit parietal, G-cells and ECL cells.

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GASTRIC SECRETION

• Let us consider Exocrine products and their role in Digestion:

1. HCL – is secreted by parietal [Oxyntic] cells in the lumen of stomach, therefore, pH in stomach lumen is below 2.

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Mechanism of HCL Secretion

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Hydrogen [H+] is actively transported in parietal cell membrane, from cell to the lumen.

Chloride [Cl-] is secreted by secondary active transport mechanism.

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Functions of Gastric Juice Functions of HCL• HCL activates pepsinogen to active enzyme pepsin.• It provides acid medium in which pepsin acts

optimally .• HCL breaks down connective tissue and muscle

fibers reducing large food particle into smaller particle.

• Denatures protein• It kills micro organism ingested with food.

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Functions of Gastric Juice Functions of Pepsinogen• Pepsinogen, an inactive enzyme produced by

chief cells. • It is stored in zymogen granules in the secretary

vesicles in the cytoplasm of chief cells.• It is released by exocytosis.• Pepsinogen is converted to active pepsin by

HCL.

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Functions of Gastric Juice

Functions of Pepsinogen• Once pepsin in formed in lumen, it acts on

pepsinogen molecule to produce more pepsin [autocatalytic or self-activating process].

• Pepsin initiates protein digestion.

• It works optimally in acidic medium.

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Pepsinogen activation in stomach lumen

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Functions of Gastric Juice Functions of Mucus• Surface of gastric mucosa is covered by mucus.• It is derived from surface epithelial cells and mucous

cells.• Mucus works as protective barrier.• It protects stomach wall from self-digestion by

pepsin.• As mucus is alkaline, pH 7 – it protects stomach from

HCL [pH 2].• Lubrication : protection from mechanical injury

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Functions of Gastric Juice Intrinsic Factor• It is secreted by parietal cells.• It is essential for absorption of vitamin B12 [vitamin B12 is

only absorbed when in combination with intrinsic factor].• Intrinsic factor – vitamin B12-complex have special

receptors in TERMINAL ILEUM, where vitamin B12 is absorbed.

• Vitamin B12 is essential for normal formation of RBC.• If no intrinsic factor, vitamin B12 is not absorbed, it

causes pernicious anemia (either in deficiency of intrinsic factor or gastrectomy).

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GASTRIC JUICE Endocrine & Paracrine cell in Gastric glands

• G-cell [Gastrin Cell] - Gastrin ( antrum )• ECL [Entero Chromaffin Cell] – Histamine ( Fundus and

body )• D-cell – Somatostatin ( pylorus antrum )• Gastrin ,Histamine, Acetylcholine [ACh] are Stimulatory

and increase HCL secretion.• Somatostatin inhabits HCL secretion.• ACh and Gastrin increase pepsinogen by acting on chief

cells.

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GASTRIC JUICE

Endocrine & Paracrine Functions• We will consider these chemical messengers in

further detail. 1. Acetylcholine – neurotransmitter released

due to vagal stimulation and in response to short local reflexes [enteric nervous system].

• ACh stimulates both parietal and chief cells, as well as, G-cells and ECL cells.

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GASTRIC JUICE2. Gastrin• Secreted by G-cells present in antrum.• Gastrin is major GIT hormone.• Gastrin is secreted in response to protein products in lumen of

stomach and in response to ACh.• Gastrin is carried by blood to the fundus and body of the

stomach and stimulates parietal and chief cells, therefore, causes secretion of HCL and pepsinogen.

• Gastrin also stimulates ECL to release histamine and increase HCL secretion.

• Gastrin is the main factor that increases HCL during meal digestion.

• Gastrin causes growth of mucosa of stomach and small intestine.

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GASTRIC JUICE3. Histamine• Is Paracrine - released from the ECL cells in

response to ACh and gastrin.• Histamine acts locally on nearby parietal cells

to increase HCL secretion.• Histamine acts via cAMP.4. Somatostatin• Released by D-cells present in the pylorus [also

in duodenum] in response to acid.• It acts locally as paracrine and inhibits

secretion of parietal cell, G-cell, and ECL cell.

Factors affecting H+ ion secretion

Alcohol & caffein also stimulate HCl secretion

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CONTROL OF GASTRIC SECRETION• It involves Three phases: 1. Cephalic Phase 2. Gastric Phase 3. Intestinal Phase

1. Cephalic Phase• It means there is increase secretion of HCL and pepsinogen

[Gastric juice before food reaches the stomach].• Stimuli are thinking, smelling, tasting, chewing and swallowing.• Increased gastric secretion occurs due to vagal nerve activity.

Vagus stimulates intrinsic plexus and increases ACh secretion, therefore, increase HCL and pepsinogen.

• Vagus also stimulates G-cells in PGA, therefore, release of gastrin which increases HCL and pepsinogen.

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CONTROL OF GASTRIC SECRETION

2. Gastric Phase• Gastric phase begins when food reaches the stomach.• Stimuli acting in the stomach are protein, caffeine,

distension, alcohol.• Protein is most potent stimulus – acts via local plexus

and vagus nerve.• Protein also stimulates G-cells to release gastrin. Important – People with hyper-acidity or peptic ulcer

should avoid caffeine and alcohol.

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CONTROL OF GASTRIC SECRETION

3. Intestinal Phase• When food enters the small intestine, it

influences gastric secretion.• Intestinal phase is inhibitory.• As Chyme enters small intestine, it reflexly

decreases the gastric secretion.

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How Gastric Secretion Decreases After It Is Produced?

• It decreases by three ways: i). As meal gradually leaves the stomach, major

stimulus that is protein in stomach is withdrawn.

ii). Somatostatin is released when food goes to duodenum. It has inhibitory effect.

iii). Stimuli which inhibit gastric motility are fat, acid, hypertonicity or distension in the duodenum.

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How Stomach Is Protected From Acidity & Enzyme Pepsin?

It is protected by :• Mucus Layer • Mucus Secreting cells [mucus pH is 7] –

secrete HCO3 which neutralizes acid [luminal pH is 2].

• Cells are impermeable to H+ ion, therefore, HCL can not penetrate into the cells.

• Entire stomach lining is replaced every 3 days.

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STOMACH

• We have discussed 1. Motility 2. Secretion in Stomach

Now we will consider 3. Digestion in Stomach 4. Absorption in Stomach

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Digestion in Stomach

• Protein digestion begins in the antrum of stomach, where food is mixed with HCL and pepsin.

• Carbohydrate digestion which started in mouth due to action of salivary amylase, continues in the stomach, till α-amylase is destroyed by the HCL.

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Absorption in Stomach

• No food or water is absorbed in the stomach.• Stomach absorbs alcohol and aspirin.• Alcohol is lipid soluble, therefore, can diffuse

through lipid membrane of epithelial cell lining of stomach.

• Alcohol is absorbed more rapidly in small intestine.

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Clinical Application Peptic Ulcer• Ulcer or erosion in the stomach, duodenum or esophagus

can occur due to increased acidity and pepsin.• In 1990, a bacterium ‘HELICOBACTER PYLORI’ was

identified as cause of peptic ulcer.• H.Pylori bacteria resides below the protective mucus

layer.• H.Pylori usually settles in the antrum which has no acid

producing parietal cells, although acid reaches the antrum.

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How H.Pylori Causes Peptic Ulcer? It causes peptic ulcer:• By toxins which cause gastritis.• By weakening gastric mucosal barrier.

OTHER FACTOR THAT CAUSE PEPTIC ULCER• Ethyl alcohol• NSAIDS [Non Steroidal Anti-Inflammatory Drugs] e.g.

aspirin, IBUPROFIN [used for arthritis, pain].• Stress – probably by increased gastric secretion.

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TREATEMENT FOR PEPTIC ULCER

• Anta-acid • H2 Histamine receptor blocker e.g. Cemitidine,

Ranitidine• Proton pump blocker [it blocks H+ - K+ ATP

pump] e.g. OMEPRAZOLE • Anti - biotic for H.Pylori

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References

• Human physiology by Lauralee Sherwood, seventh edition

• Text book physiology by Guyton &Hall,11th edition

• Text book of physiology by Linda .s contanzo,third edition